Improve maneuver node execution accuracy

Use naive estimation of burn duration to advance execution by
appropriate amount of time. Could be improved by taking into account
change in vessel mass during burn.

Use maneuver node to determine remaining delta-v for burn. This seems
more accurate than calculating accelation since last tick.

Throttle down to 50% when within 1m/s of target delta-v and terminate
when 0.1m/s is left on maneuver.

These changes reduce final burn discrepancies from >2m/s to ~0.1m/s
for a 3 minute transfer from 100km Kerbin orbit to a 10km Minmus
encounter.

I'd been keen for others to test this on longer/shorter burns to see
how they get on.